Digital filter banks are useful for performing certain selective operations on a broadband signal, such as for spectral analysis and for telephone communications. In telephone communications, for example, it is frequently necessary to provide signal conversion between a TDM (time-division multiplexed) signal format and an FDM (frequency-division multiplexed) signal format. This can be accomplished in each direction in known ways by a signal translator incorporating a digital filter bank of appropriate design for effecting the conversion.
A digital filter bank can be constructed of a transform section connected to a polyphase network of digital filters. A polyphase network allows all the operations to be done at the input bit rate and only then multiplexed to a higher rate. Both such sections can be provided by one or more appropriately adapted large scale integrated circuits known as "digital signal processors" which each contain on a single chip a large number of memory elements and an arithmetic unit in the form of a multiplier-accumulator for operating on values from the memory elements in a selected mode.
Since each digital signal processor generally has only a single multiplier-accumulator available for operating on the values from the memory, it is important to make efficient use of it. Additionally, it is highly desirable to minimize the number of times that the contents of the accumulator of the multiplier-accumulator must be cleared and transferred to the memory each sampling period, because each such operation tends to introduce round-off noise in the signal.